1. Field of the Invention
The present invention generally relates to a power-saving circuit for an electronic device. More specifically, the present invention relates to a power-saving circuit applied to the power supply circuit of a monitor. When the monitor is in a power-saving mode, this power-saving circuit can automatically turn off the power supply fed to the voltage booster of the monitor for reducing the power consumption of the monitor.
2. Description of the Related Art
In order to decrease wasted power, most computers in the current market can automatically detect usage/non-usage and enter a power-saving mode if required. Generally speaking, the power-saving mode can be further divided into several finer sub-modes. For the sake of clarity, however, only two modes, the normal mode and the power-saving mode, are discussed.
When a computer is set to enter the power-saving mode, the monitor of the computer is also notified. While in the power-saving mode, the monitor only provides power supply to essential circuitry that is used to continuously receive instructions from the computer and to quickly recover the normal mode when receiving the corresponding instruction.
Referring to FIG. 1 (Prior Art), an ordinary computer system includes a computer mainframe 20, a monitor 28 and other accessories. Computer mainframe 20 can detect the usage condition and decide, according to the detection result, whether or not the computer system enters the power-saving mode. When deciding to enter the power-saving mode, computer mainframe 20 does not only enter into power-saving mode itself, but also notifies monitor 28 to enter the power-saving mode.
The modern monitor usually includes a voltage booster at the entry point of the external power supply for dealing with different voltage specifications. The voltage booster is mainly comprised of booster controllers, triac devices and other related components. In addition, the voltage booster can detect the alternative current (AC) voltage supplied by the external power supply and selectively boost the AC voltage according to the detection result. For example, suppose that the internal circuit of a monitor is designed for 200.about.220 VAC. When detecting that the external power supply is 200.about.220 VAC, the voltage booster can directly connect the external power supply to the internal circuit. When detecting that the external power supply is 100.about.110 VAC, the voltage booster can boost the external power supply and then connect it to the internal circuit. Using the boosting scheme, this monitor can be used in various environments.
FIG. 2 (Prior Art) is a circuit diagram of the power supply circuit applied in the conventional monitor. As shown in FIG. 2, the power supply circuit includes a voltage booster 320, a first rectifier 310, a second rectifier 300 and a first power-saving circuit 370. For the sake of clarity, FIG. 2 only illustrates a partial circuit that provides 15 VDC as a second rectifier 300 located in the secondary winding of transformer T1. In reality, there are several different circuits in the secondary winding for providing different voltages, such as -15V and 5V. IC1 represents a booster controller chip, which is used to detect the amplitude of the external power supply and decide whether the triac chip IC4 should perform the boosting operation.
Suppose that the monitor in FIG. 2 is designed for 200.about.220 VAC. The booster controller chip IC1 can be implemented by AVS1AC or AVS1BC developed by SGS-Thomson and the triac chip IC4 can be implemented by AVS08CB developed by SGS-Thomson. Nodes AC1 and AC2 are connected to two terminals of the external AC power supply, respectively. When detecting that the external power supply is 200.about.220 VAC, the booster controller chip IC1 disables the triac chip IC4, and nodes A1 and A2 of the triac chip IC4 are open-circuited. Therefore, the voltage on node A is about 200.times.2.about.220.times.2. On the other hand, when detecting that the external power supply is 100.about.110 VAC, the booster controller chip IC1 enables the triac chip IC4, and nodes A1 and A2 of the triac chip IC4 are close-circuited. Therefore, the voltage on node A can be boosted from the original voltage 100.times.2.about.110.times.2 to 200.times.2.about.220.times.2. Using such scheme, the potential of node A can be maintained in 200.times.2.about.220.times.2 in different operating environments. Accordingly, the monitor can be normally operated.
A mode-detection chip IC3 is used to receive instructions indicating the current mode from computer mainframe 20. When receiving an instruction indicating the normal mode, the mode-detection chip IC3 outputs a logic high signal to turn on transistor Q3 and transistor Q4. Then second rectifier 300 can provide power to the internal circuit of the monitor 28. When receiving an instruction indicating the power-saving mode, the mode-detection chip IC3 outputs a logic low signal to turn off transistor Q3 and transistor Q4. Then the second rectifier 300 stops providing power to the internal circuit of the monitor 28. Therefore, the conventional monitor saves power by stopping the power supply connected to the secondary windings of the transformer T1. However, since the voltage booster 320 is directly connected to the external AC power supply, it is inevitable that the voltage booster 320 still consumes a certain amount of power. In fact, the power consumption of the voltage booster 320 is quite large.